Nitrogen (N) is the most abundant inorganic nutrient taken up from the soil by plants for growth and development. Maize roots absorb most of the N from the soil in the form of nitrate, the majority of which is transported to the leaf for reduction and assimilation. Nitrate is reduced to nitrite by nitrate reductase (NR) in the cytosol and then nitrite is transported into chloroplast where it is reduced by nitrite reductase (NiR) to ammonium. Ammonium is assimilated into glutamine by the glutamine synthase-glutamate synthase system (Crawford and Glass, (1998) Trends in Plant Science 3:389-395.). Also, it has long been known that significant amounts of N are lost from the plant aerial parts by volatilization (Glyan'ko, et al., (1980) Agrokhimiya 8:19-26; Hooker, et al., (1980) Agronomy Journal 72(5):789-792; Silva, et al., (1981) Crop Science 21(6): 913-916; Stutte, et al., (1981) Crop Science 21(4):596-600; Foster, et al., (1986) Annals of Botany 57(3):305-307; Parton, et al., (1988) Agronomy Journal 80(3):419-425; Kamiji, et al., (1989) Japanese Journal of Crop Science 58(1):140-142; Morgan, et al., (1989) Crop Science 29(3):726-731; O'Deen, (1989) Agronomy Journal 81(6):980-985; Guindo, et al., (1994) Arkansas Farm Research 43(1):12-13; Heckathorn, et al., (1995) Oecologia 101(3):361-365; Cabezas, et al., (1997) Revista Brasileira de Ciencia do Solo 21(3):481-487). Experimental evidence supports the loss of N through ammonium and not through N oxides (Hooker, et al., 1980). Treatment with chemicals that inhibit glutamine or glutamate synthase activities led to increased loss of ammonium through volatilization (Foster, et al., 1986). Loss of N is not only limited to C-3 species as C-4 plants have also been reported to lose N through volatilization (Heckathorn, et al., 1995).
Manipulation of AMTs can be utilized to improve NUE by causing increased dry matter, thereby contributing to an increase in plant yield. Two of the ways to improved dry matter accumulation are: 1) reduce N loss through volatilization and 2) reduce N content of the plant so that more dry matter can be accumulated in the form of low-energy constituents, e.g., starch or cellulose.
For ammonium to be lost from the leaf, it must first pass through a facilitated channel since it is highly hydrophilic. Ammonium transporters (AMTs) were originally discovered as ammonium transporters but some recent studies have shown that at least in some cases AMTs can act as gas channels (Soupene, et al., (2002) Proc Natl Aced Sci USA 99:3926-3931; Kustu and Inwood, (2006) Transfus Clin Biol 13:103-110). An amtB knock-out mutant of Salmonella grows better on poor N source, apparently because it can sequester more N by keeping it from leaking back out (Soupene, et al., 2002). This application details an invention which is used to manipulate AMTs in higher plants to improve NUE. The inventors identified chloroplast-specific and/or leaf-preferred AMT(s) and knocked them out/down to minimize the loss of ammonium, which resulting in better N assimilation/NUE. In addition, work was not limited only to the chloroplast-localized AMTs but will also down-regulation of the AMTs that are localized to other organelles/membranes.